The rock crab (Cancer irroratus) fishery is a growing industry in eastern Canada. Considering that American lobster (Homarus americanus) is highly dependent on the rock crab as a food source, questions have arisen as to the impacts such a fishery would have. This study examines how different rations of rock crab can affect somatic growth, condition and ovary development of mature lobster, following molt. We tested the effect of four diets containing various amounts of rock crab, blue mussel and green sea urchin. The four diets were: a reference diet where 80% of the energy was provided by rock crab (T), a diet with half the crab content of the reference diet but containing as much protein (isoproteinic) as the reference diet (E1), a diet without crab but isoproteinic with the reference diet (E2), and a diet without crab but with as much energy (isocaloric) as the reference diet (E3). In general, lobsters fed a diet without rock crab showed lower glycogen and lipid content and higher water content in the digestive gland. Growth of chela muscles was lower, although diet did not have any effect on protein concentration. Ovary development was stunted in females. Differences were mostly striking in diet E3, which contained less proteins than the reference diet. Results obtained from diet E2 were also significantly different from the reference diet and not from E3, suggesting that mussel and urchin, even if given in a greater amount, are not equivalent to crab and cannot fully compensate the absence of this essential component of the lobster's diet. The importance of rock crab for lobster may be due to its high protein content and presence of particular amino acids. Our results strongly suggest that the development and management of a rock crab fishery should be cautious and governed by a multi-species approach.